Have also heard tales of people turning down the shank even smaller on a barrel, then maching a threaded sleeve that fits very tightly screwed on the shank, and the its threaded to the 03 action..?? I was told if the new threaded collar/bushing had very good thread contact, it would be as strong, or close to an original untouched shank..any ideas? Thanks.

Some Italian Carcanos were converted to .308 Winchester using this method. The conversions were done by a UK company and the rifles sold as target rifles or for hunting.
A 7X57 conversion of a 1903 should be okay if done properly.

Some WW1 surplus Gew98 rifles with corroded bores were bored out and fitted with a 7X57 liner. Don't know much about those.